Many systems require redundant sources of electrical power in order to insure correct operation even when one or more power sources may fail during operation. Computer systems, for example, are particularly sensitive to power supply interruptions of any duration. Such systems are now being designed so as to be tolerant of the failure of one or more hardware components of the system before the system as a whole fails and must be shut down. Consequently, the power delivered to such computer systems must also have a similar degree of fault tolerance.
Fault-tolerant systems are particularly necessary in systems used in applications in which a failure could have diasastrous results, as in systems controlling the operation of spacecraft and satellites which systems, when failure occurs, are very difficult or, in some cases, substantially impossible to repair. In other systems, failure of a computer portion thereof may cause damage to other parts of the system with which it is interconnected. A prime example of such a problem arises in systems for controlling nuclear reactors. Further, high performance aircraft, for example, are inherently unstable and cannot be directly controlled by the pilot and so a fault-tolerant flight control system for such aircraft becomes a necessity.
In current fault-tolerant power distribution systems in which a plurality of redundant power sources are normally used, if one source fails the others can be utilized to make up the failure and to provide the necessary power for the system. A common practice in this regard is to use "diode-or" circuitry which interconnects two or more power sources to each load. Such an approach has disadvantages, however, particularly when dealing with relatively low voltage power sources, such as the common 5-volt power supply used in microprocessor systems. Diodes have forward voltage drops which may approach a substantial portion, e.g., up to 20% in some cases, of the supply voltage itself and may represent a significant waste of power in such systems. Such voltage drops vary with both load current and temperature and the presence of large changes in load current or temperature may, therefore, cause the supply voltage to exceed the required tolerance levels.
In addition, where a large number of loads and power sources are utilized, the use of diode-or circuitry may require large number of additional wires in order to distribute the power throughout the power source/load network.
A further disadvantage of a simple diode-or configuration is that a short to ground at one load can bring down the power supplied to all loads unless some form of overcurrent protection is employed. Typical designs use fuses for this purpose, which have to be manually replaced in event of opening.
Such disadvantages make the simple diode-or circuit approach less desirable in many applications and in some cases effectively unusable. Hence, it is desirable that a better approach to the redundant power distribution problem be devised to prevent the disadvantages of wasted power and excessive wiring requirements of the diode-or technique.